The recommended replacement is 100,000 miles. I've seen some as high as 140,000, but wouldn't recommend waiting that long.

Climate shouldn't have much impact on it, as it is pretty much a sealed system. There are some rubber spacers that I suppose could let dust or water in, especially if they are damaged somehow. I live in Houston (Hot and Hotter) in an area with lots of dust because of construction, but didn't see any abnormal wear at around 120,000.

 

From what i've read, and what everyone before me has said, it has probably bent valves. But just incase, just pull the cam covers off and check valve clearances before you pull the heads off. If valve clearances are all in spec (somehow), then put it back together. If valves are bent, cam/bucket clearance will be way more than spec (Intake 0.15–0.25 mm - Exhaust 0.25–0.35 mm). You never know your luck, but i wouldn't bank on it

 

odds are good that you'll have to pull the heads and have new valves and guides installed at best. at worst, the pistons have holes in them also from teh impact. depends on how fast the engine was spinning when the belt let go.

 

first of all u need to put a new belt back on and check compression,more than likely it has bent valves and has damaged the top of some of the pistons, i have however seen a belt blow on an interference motor with no damage,but its highly unlikely...recommended mileage is 90k or 7 years

 

from what i've read on the forums, 90k-100k miles is the way to go (while you're at it, along with water pump, tensioner, etc...), even though some owners have gone past 100k with the same T-belt.
for the interference engines, better safe than sorry and after the replacement you know your good to go for quite some time.

good luck to you with getting everything sorted out